hypermedia-transitions

This module is designed to help you to add hypermedia support for you API.

Features

This module consists of :

• a fixed data structure to store your API state transitions (more details in the next section)
• parsers for many different hypermedia-compliant media types exploiting this data structure to build the response.

Use as Express middleware

In your Express file, you need to add a couple lines :

• setup all your state transitions using the "addTransition" function
• register the interceptor in your middlewares.

All set ! If no header is specified, your API responses won't change. But if a client requires a particular format using the "Accept" header, and if you chose to support it, he will receive it in the response !

Example :

 
//
// state_transitions.js 
//
 
const list_transitions = [
  {
    rel: "resource_list", 
    target: "resource list",
    accessibleFrom: [{ state: "home" }],
    href: "/resources",
    method: "get"
  },
  {
    rel: "resource", 
    target: "resource",
    accessibleFrom: [{ state: "resource list" }],
    href: "/resources/{id}",
    isUrlTemplate: true,
    method: "get"
  },
  {
    rel: "resource_delete", 
    target: "resource",
    accessibleFrom: [{ state: "resource list" }],
    href: "/resources/{id}",
    isUrlTemplate: true,
    method: "delete",
    authRequired: true
  }
]
 
exports.listTr = { list_transitions }
 
//
// app.js
//
 
const express = require('express')
const transitions = require('hypermedia-transitions')
const listTr  = require('./state_transitions').listTr
 
var app = express()
 
// add the middleware that need to be set early 
 
transitions.addTransitionList(listTr)
 
app.use(transitions.halInterceptor)
 
// define your routes, your error handlers, and start your server. 
 

Use as HAPI plugin

 
const hypermediaOptions = {
  mediaTypes: ['hal', 'siren'],
  transitions: require('../list_transitions.json')
}
 
server.register([{
    register: require('../hypermedia-transitions').hapiRegister,
    options: hypermediaOptions
  }, {
// ...
}
], function (err) {
    if (err) { return console.log(err); }
 
    server.route(require('./routes'));
 
    server.start(function () {
      console.log('API up and running at:', server.info.uri);
    });
});

Data structure

The state transitions you define should be objects defining the following properties :

• rel : the name for your transition

Warning : in some formats, the "rel" attribute becomes a Javascript object key. Hence, using characters such as "." or "-" can cause an error.

• target : the target state of your transition
• accessibleFrom : a list of objects describing how your transition can be triggered (more details in next section)
• href : the URL to trigger your transition (relative from domain name)
• isUrlTemplate : whether the URL written in the previous field can be used "as is" or is a URL template that needs to be filled in
• method : the HTTP method to trigger your transition
• authRequired : whether or not the client needs to be authenticated to be able to trigger that transition
• template : a template for what kind of data should be sent (POST or PUT methods for example)

Example :

{
  rel: "update_task",
  target: "task",
  accessibleFrom: [
    { state: "home" }, 
    { state: "task list" },
    { state: "task", fillTemplateWith: {task_id: "id"} }
  ],
  href: "/tasks/{task_id}",
  isUrlTemplate: true,
  method: "post",
  authRequired: false,
  template: {
    name: "string",
    completed: "bool",
    description: "string"
  }
}
 
// "id" here is a data element that is included in your API response when displaying a task resource

Objects in "accessibleFrom" list

They can have the following properties:

• state : the state from which it can be triggered (required)
• fillTemplateWith : a dictionnary describing how to fill the URL template, when the transition is available from this state, using data included in your response. Must be formatted this way :

{ url_template_parameter: "data_corresponding_parameter" }

• eachItem : set to true if your data is a list of elements and the URL template must be filled with a different value for each element. Example :

{
// ...
  accessibleFrom: [{state: "resource list", fillTemplateWith: {id: "id"}, eachItem: true}]
// ...
}
 
// Will result in (HAL for example) : 
 
[
  { 
    // ...
    _links: {
      resource: {
        href: "http://example.org/resources/1"
      }
    }
  },
  { 
    // ...
    _links: {
      resource: {
        href: "http://example.org/resources/2"
      }
    }
  }
]
  

• withSelfRel : You can specify this attribute if you want a link to be the "self" relationship, in embedded resources or not. Example :

{
// ...
  accessibleFrom: [{state: "list resources", fillTemplateWith: {id: "id"}, eachItem: true, withSelfRel: true}]
// ...
}
 
// Will result in (HAL for example) : 
 
[
  { 
    // ...
    _links: {
      self: {
        href: "http://example.org/resources/1"
      }
    }
  },
  { 
    // ...
    _links: {
      self: {
        href: "http://example.org/resources/2"
      }
    }
  }
]
  

"state" and "target"

The "state" property from the "accessibleFrom" objects and the "target" property from the transition work together. When a transition is triggered, your "state" become its "target", and this state is used to figure out which other transition are available. I advise you to write down a graph representing your API state and transitions to be sure not to have forgotten any. Your state names will most of the times consist in resources names. For naming consistency, put the resource name in first position and before a space or a "_" in the "state" string since it is used in parsing for naming lists. Example in HAL :

// Current state : "task_list"
// Original API data :
[ 
  //... some objects ... 
]
 
// Becomes in HAL : 
{ _embedded: {
  task: [
    // objects...
    ]
  }
}

Authentication

To be able to display results that are only visible by authenticated users, you will have to add a req.isAuth property. If it is not set or false, the translators will consider that the user is not authenticated.

Warning : for this to work you must verify if your user is authenticated for EACH request and a client that requires authentication for only some actions MUST authenticate for each request. Otherwise the transitions that are "authentication-protected" won't be visible.

Featured media types

• HAL (http://stateless.co/hal_specification.html)
• Siren (https://github.com/kevinswiber/siren)
• ... more to come ! Don't hesitate to submit pull requests with your own translators for other hypermedia types.